1. Field of the Invention
This invention relates generally to offshore oil or gas production, oil or gas storage, and offloading and deepwater floating structures used in such applications, and more particularly a floating offshore vertical caisson storage platform structure with a telescoping keel tank for use in oil drilling and production and oil or gas storage operations.
2. Background Art
The purpose of offshore oil development is to build reliable structures to produce and to transport oil and gas. The cost and time of these developments and the risk and consequences of failure are crucial to the petroleum industry. The successes of several innovations, on the other hand, are very visible in the past two decades. Petroleum companies are very active in cost and time reduction through research and development and reengineering.
Several offshore fields around the world are pipeline dominated in the oil transportation from the offshore platform to the land based facilities. Subsea production systems with an assembly of pipes, structures, equipment and valves are further developed from past experiences of the sea-bottom pipeline technology. Industry is currently investigating the feasibility of building a seabed processing unit with piping, valves and instrumentation pumps and control systems. It may be feasible to do the entire production operation with ever growing sea-bottom technology. However, the cost and reliability of such developments with every thing on the seabed is difficult to prove.
The seabed pipeline system is expensive in the case of deepwater, in the case of poor seabed condition and also in the case of the production facility situated very far from the shore. Ship-shaped surface vessels are commonly used for oil storage.
The present independent vertical caisson fluid storage structure has all of the advantages of a SPAR and Tension Leg Platform (TLP) structure, and eliminates the need for connections to the subsea pipelines on the seabed, and provides a viable transportation alternative to pipelines. The present structure makes use of independent oil or gas production and independent oil or gas transportation, and the vertical floating caisson structure with a telescopic keel tank allows it to be positioned permanently moored for tandem above-water loading with respect to production platform and shuttle export tankers stationed on the other side of the storage structure. Tandem above-water offloading is established between the export vessel and storage caisson by quick-connect hoses, and quick connect hawsers are used between the production, storage and export units.
Independent vertical caisson oil or gas storage is efficient to accommodate large storage capacity such that continuous production is possible without interruption due to the failure of the tanker arrival in severe weather conditions. Also, the production rate is much slower compared to the export tanker capacity and thus the tanker waiting time is reduced in the offloading process.
Vertical storage also has significant advantages over the ship-shaped surface vessel storage. The ship-shape storage vessels are sensitive to the sea-surface condition and oscillate largely with surface waves. Resonant sloshing of stored oil or gas poses danger to the structure. The vertical caisson structure of the present invention, on the other hand, has minimum vertical motion for the surface waves.
Lloyd, III, U.S. Pat. RE No. 29,167 discloses a twin hull variable draft drilling vessel having a pair of laterally spaced elongated hulls with a plurality of upstanding columns spaced therealong supporting a working platform in spaced relation above the hulls a distance slightly greater than the maximum anticipated wave height. The hulls buoyantly support the vessel in a low draft floating condition with the hulls having freeboard. The hulls have ballast compartments to submerge the hulls and portions of the stabilizing columns to a distance of approximately half the effective height of the stabilizing columns, which is slightly greater than maximum anticipated wave height, to maintain the vessel in a high draft floating condition with the platform elevated above the waterline. The columns stabilize the vessel in the high draft condition about roll and pitch axes. The working platform mounts either a drilling rig or a heavy duty crane or like operational equipment along the centerline of the vessel.
Goren et al, U.S. Pat. No. 4,232,625 discloses a Column stabilized semi-submerged drilling vessel having a pair of laterally spaced elongated hulls with a pair of upstanding columns at opposite ends thereof supporting a working platform and a drilling mast in spaced relation above the hulls. The hulls buoyantly support the vessel in a low draft in-transit floating condition with the hulls having freeboard. The hulls have ballast compartments to submerge the hulls and portions of stabilizing columns such that the waterline lies intermediate the height of the columns with the platform and drilling mast elevated above the waterline. The columns stabilize the vessel in the high draft condition and the number, cross-sectional area and configuration of the columns, the weight distribution of the vessel and the geometry of the submerged hulls and portions of the columns are such that the vessel obtains motion minimizing characteristics in the high draft condition. Various features include hydrostatic and geometric properties wherein the ratio of the righting moment about the pitch axis to the righting moment about the roll axis in high draft condition is within a range of 1.0 to 1.3 while the ratio of vessel length to width lies within a range of 1.2 to 1.5; a natural period in heave in a range of 16–18 seconds; and a heave response amplitude operator in a range of about 0.35 to 0.60 in the range of waves of 10–14 seconds.
Thomas, U.S. Pat. No. 6,024,040 discloses a mobile jack-up platform converted to a semi-submersible offshore platform. The platform includes single submerged hollow lower base at the bottom end, partially submerged elongate vertical buoyant connecting legs extending upwardly therefrom and passing through an upper barge (jack-up platform) above the level of the sea. The hollow base has a square, rectangular or triangular configuration and is filled with seawater to form the ballast for the entire platform and may include interior reservoirs in which hydrocarbons are stored. A central opening or passage in the center of the base reduces the resistive surface size of the base in the water during vertical movements of the platform. The vertical connecting legs have a hollow cylindrical upper portion with a bottom wall forming a buoyancy tank, and a lower portion formed of open frame lattice construction. The respective lengths of the hollow cylindrical buoyant upper portion and the lattice-work lower portions are dimensioned relative to one another so that a pressure force exerted by the sea on the upper portions substantially compensates for an acceleration force exerted on the base by the action of the seawater surrounding the base over a usual swell period range of the sea. The platform in operation imitates a semi-submersible, which can retract the legs with respect to the upper barge (jack-up platform). With the legs fully retracted above the upper barge and its single lower base closely adjacent to the bottom of the upper barge, the platform can be floated and transported to another location.
Horton, U.S. Pat. No. 5,558,467 discloses a spar-type deep water offshore floating apparatus for use in oil drilling and production in which an upper buoyant hull of prismatic shape is provided with a passage longitudinally extending through the hull in which risers run down to the sea floor, the bottom of the hull being located at a selected depth dependent upon the wind, wave, and current environment at the well site, which significantly reduces the wave forces acting on the bottom of the hull, a frame structure connected to the hull bottom and extending downwardly and comprising a plurality of vertically arranged bays defined by vertically spaced horizontal water entrapment plates and providing open windows around the periphery of the frame structure, the windows providing transparency to ocean currents and to wave motion in a horizontal direction to reduce drag, the vertical space between the plates corresponding to the width of the bay window, the frame structure being below significant wave action whereby wave action thereat does not contribute to heave motion of the apparatus but inhibits heave motion, the frame structure serving to modify the natural period and stability of the apparatus to minimize heave, pitch, and roll motions of the apparatus. A keel assembly at the bottom of the frame structure has ballast chambers for enabling the apparatus to float horizontally and for stabilization of the apparatus against tilting in vertical position, and taut anchor lines connected to the apparatus at a location of relatively little cyclic movement of the apparatus, the said lines being connected to suitable anchors.
Horton, III, U.S. Pat. No. 5,722,797 discloses a spar-type buoyant floating caisson for offshore drilling and production that includes means for increasing the natural period of the caisson and reducing heave, pitch, and roll without increasing the overall length of the caisson. The floating caisson has a center well through which drilling and/or production risers pass and one or more circular plates extend radially from the caisson below the water surface. The circular plates provide additional mass and resistance to environmentally induced motions and thus increases the natural period of the caisson beyond the periods of maximum wave energy, which allows the caisson to be designed with a shallower draft than a caisson without the plates that would normally be used in deep water.
Blevins et al, U.S. Pat. No. 6,206,614 discloses a spar-type floating offshore drilling/producing structure that is formed from a plurality of closely spaced vertically oriented buoyant columns on which one or more modules or decks may be placed to support process equipment, a drilling rig, utilities, and accommodations for personnel. The columns are held in the closely spaced relationship by a plurality of horizontal plates spaced along the length of the columns and vertical plates located near the bottom of the columns and near the top of the columns. The columns have a smaller water plane area than the horizontal plates. The structure includes fixed ballast, an oil storage area, and voids and variable ballast for offsetting the lighter weight of the stored oil.
Xu et al, U.S. Pat. No. 6,652,192 discloses a heave suppressed, floating offshore drilling and production platform having vertical columns, lateral trusses connecting adjacent columns, a deep-submerged horizontal plate supported from the bottom of the columns by vertical truss legs, and a topside deck supported by the columns. The lateral trusses connect adjacent columns near their lower end to enhance the structural integrity of the platform. During the launch of the platform and towing in relatively shallow water, the truss legs are stowed in shafts within each column, and the plate is carried just below the lower ends of the columns. After the platform has been floated to the deep water drilling and production site, the truss legs are lowered from the column shafts to lower the plate to a deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform. Water in the column shafts is then removed for buoyantly lifting the platform so that the deck is at the desired elevation above the water surface.
The present invention is distinguished over the prior art in general, and these patents in particular by a floating offshore vertical caisson fluid (oil or gas) storage platform having a large diameter vertically oriented buoyant column or caisson, or multiple caissons, defining a storage chamber, and a telescopic keel tank disposed at the bottom end thereof, and may have deck on top of the caisson to support storage, drilling and production structures, equipment, and quarters. The structure can be transported horizontally either dry on a transporting vessel or towed with its keel tank in a fully retracted position. At the field of operation, the structure initially floats horizontally. The keel tank is extended and then slowly flooded to move the center of gravity of the structure toward the keel tank and with the heavier tank, the structure tilts upright to assume an operating vertical position with the telescopic keel tank extended downward with respect to the caisson, and thereafter as the storage chamber is filled with fluid, the relative position of the keel tank is adjustably tuned to raise or lower the center of gravity of the entire mass of the structure with respect to its center of buoyancy and maintain the center of gravity of the structure below its center of buoyancy and stabilize the structure vertically at a desired draft according to ballast and variable or fixed loads, and to compensate for different operational, environmental and installation stages of the structure.
The telescopic spindle and keel tank of the present structure is used both in the installation of the caisson and also in the storage operation. In the loading and offloading operation, the center of gravity of the storage caisson structure will move significantly in the vertical direction. The keel tank is efficiently telescoped up and down and also is partially ballasted to maintain the center of gravity below the center buoyancy of the structure. Cantilevered booms situated on top of the deck facilitate quick connect and disconnect of loading and offloading hoses and maintain them completely above the water during the loading and offloading operation between the floating storage platform and a production platform and export shuttle tanker.